The present invention relates to microlenses, and more particularly, to liquid microlenses.
Lasers, photoconductors, and other optical components are widely used in many optoelectronic applications such as, for example, optical communications systems. Traditionally in such applications, manual positioning and tuning is required to maintain the desired optical coupling between the system components. However, such manual positioning can be slow and quite expensive.
More recently, in attempts to eliminate this manual positioning, small tunable lenses (also known as tunable microlenses) were developed to achieve optimal optical coupling. Typically, these microlenses are placed between an optical signal transmitter, such as a laser, and an optical signal receiver, such as a photodetector. The microlens acts to focus the optical signal (e.g., that is emitted by the laser) onto its intended destination (e.g., the photodetector). In some cases the refraction index of these microlenses is automatically varied in order to change the focus characteristics of the microlens when the incidence of a light beam upon the microlens varies from its nominal, aligned incidence. Thus, the desired coupling is maintained between components of the microlens. Therefore, the manual positioning and adjustment required in previous systems is eliminated.
Most tunable microlenses are either gradient index (GRIN) lenses with the refractive index controlled electrostatically or flexible polymeric lenses with the shape (and, therefore, the focal length) controlled mechanically. Both technologies have inherent limitations that impose severe restrictions on the performance of these existing tunable microlenses.
Tunable gradient index lenses have inherent limitations associated with the relatively small electro-optic coefficients found in the majority of electrooptic materials. This results in a small optical path modulation and, therefore, requires thick lenses or very high voltages to be employed. In addition, many electro-optic materials show strong birefringence that causes polarization dependence of the microlens, which distorts light with certain polarization.
Mechanically adjustable flexible lenses typically have a substantially wider range of tunability than the gradient index lenses. However, they require external actuation devices, such as micropumps, to operate. Integration of such actuation devices into optoelectronic packages involves substantial problems associated with their miniaturization and positioning. These become especially severe in the case where a two-dimensional array of tunable microlenses is required.
Attempts have also been made to use other technologies to produce tunable microlenses, such as liquid microlenses controlled through self-assembled monolayers. Some of these attempts are described in U.S. Pat. No. 6,014,259, issued Jan. 11, 2000, the entirety of which is hereby incorporated by reference herein. Microlenses utilizing self-assembled monolayers, however, also suffer from several problems, including severe limitations on material selection and strong hysteresis often leading to the failure of the microlens to return to an original shape after a tuning voltage is disconnected.
None of the above-described microlenses allow for both lens position adjustment and focal length tuning. Therefore, more recent attempts have involved developing liquid microlenses that permit such lens position and focal length adjustments. Examples of such microlenses, which utilize electrowetting principles coupled with external electronic control systems to accomplish these adjustments, are described in Applicants""copending U.S. patent applications Ser. No. 09/884,605 now U.S. Pat. No. 6,538,823 filed Jun. 19, 2001, entitled xe2x80x9cTunable Liquid Microlensxe2x80x9d and Ser. No. 09/951,637 now U.S. Pat. No. 6,545,815 filed Sep. 13, 2001, entitled xe2x80x9cTunable Liquid Microlens With Lubrication Assisted Electrowetting.xe2x80x9d
We have recognized that, while the ""605 and ""637 applications provide exemplary electrowetting-based tunable liquid microlenses, there remains a need to provide a tunable liquid microlens that does not rely on an external electronic control system to detect out of alignment conditions and adjust the position and/or focal length of the microlens. In particular, in certain applications it may be advantageous to have a microlens that is self-tunable. Such a microlens would eliminate the cost and effort associated with integrating the microlens control electronics previously necessary to tune electrowetting-based microlenses and would potentially reduce the tuning time.
Therefore, we have invented a microlens that uses a layer of photo-conducting material (such as a conjugated polymer, a doped charge transporting polymer, or certain inorganic semiconductors) to create a voltage differential between at least one of a plurality of electrodes and a droplet of conducting liquid. Such a droplet, which forms the optics of the microlens, will move toward an electrode with a higher voltage relative to other electrodes in the microlens.
One embodiment of such a self-tunable microlens comprises a transparent conducting substrate of a material (such as transparent glass) that is transparent to at least one wavelength of light useful in an optical system. A plurality of electrodes is disposed on the aforementioned photo-conducting material in a way such that they may be selectively biased to create a respective voltage potential between the droplet and each of the plurality of electrodes. The photo-conducting material is, in turn, disposed on the transparent conducting substrate between the light beam source and the plurality of electrodes. A layer of dielectric insulating material separates the plurality of electrodes and the photo-conducting material from the droplet of conducting liquid.
When light is incident upon the photo-conducting material, a leakage current results. When a light beam is equally incident on the photo-conducting material associated with each electrode in the layer of electrodes, the leakage current through each electrode is equal and the droplet remains in its initial, centered position. However, when the light beam becomes misaligned with the electrode pattern such that it is incident more upon one segment of photoconducting material than the others, a greater leakage current develops in that segment than otherwise would be present when the light beam is incident equally upon all segments. This greater current also causes the voltage across the electrode associated with that segment to decrease. An electrical circuit coupled with each electrode detects this change in current (or voltage) and then adjusts the voltages applied to each electrode in such a manner as to ensure that a higher voltage is applied to the electrode(s) toward which the droplet must move in order for the microlens to be aligned with the light beam.
In another embodiment of the present invention, the microlens requires no electrical circuit to adjust the voltages across the electrodes to achieve the droplet""s desired location. Instead, two layers of electrodes are used, an upper layer and a lower layer. Each electrode in the lower layer of electrodes is electrically coupled to the electrode in the upper layer directly opposed to that electrode in the lower layer. Thus, as described above, when a light beam becomes more incident upon the photo-conducting layer of material associated with one electrode in the lower layer, the larger leakage current through this electrode develops and, as a result, the voltage across that electrode drops. The result is that the voltage also drops in the opposing electrode in the upper layer to which that electrode in the lower layer is connected. The resulting voltage differential between the droplet and the electrodes in the upper layer is such that the droplet moves automatically toward the lower layer electrode with the lowest voltage (i.e., toward the position of greatest incidence with the light beam).